We considered the requirement “SUM formula support for time
in ejSpreadsheet” as a bug.We have
logged a report on this and will fix the issue internally, also the fix will be
included in “Vol 3 Service Pack 2 details in which the fix will be included”,
which has been scheduled to be rolled out in the month of “November 2015”.

We have also created an incident internally to have a follow
up with this issue. Please refer the below incident link,